Carrageenan is a complex mixture of sulfated polysaccharides comprising linear polymers of 1,3 bound α-D-galactose units and 1,4 bound α-D-galactose units with the general formula:→3α)A(1→4β)B(1→3α)A(1→4β)B(1→3α)A(1→4β)B(1→wherein A and B represent galactose residues derived from two different groups.
Carrageenan is extracted from red seaweeds and constitutes the principal structure of the seaweed. It is located within the cell wall and intracellular matrix of the plant tissue. The carrageenan content of commercially harvested seaweeds is generally between 30% and 80% based on the seaweed dry weight.
Carrageenan finds wide applicability as a food ingredient and is functional in foods such as dairy products, water dessert gels, meat products, confections, beverages, dressings and other such products. Carrageenan is also used in non-food products such as cosmetics, toothpaste, and other personal care products. The molecular weight of commercial carrageenan products is typically from about 100,000 to 1,000,000 Daltons. Carrageenans have the unique ability to form an almost infinite variety of gels at room temperature, with a variety of gelling and melting points. These gels require no refrigeration, and can be made stable through repeated freeze-thaw cycles. Carrageenan solutions can thicken, suspend, and stabilize particulates, colloidal dispersions and water/oil emulsions. The solutions shear thin, which allows them to be pumped easily. Also, the solutions rapidly rebuild viscosity and suspending power on standing. Depending upon the food application, carrageenan present in a few percent by weight or less provides gelling, thickening, and binding, and also helps in imparting appropriate texture to the food product. Owing to these qualities, carrageenan is particularly desirable as a fat replacement in low fat foods.
Carrageenan is generally soluble in warm water, in which it forms viscous solutions. It is insoluble in most organic solvents, and typically forms complexes with proteins. The major types of carrageenan are designated as kappa, iota, lambda, nu and mu. These are differentiated based on the nature of the repeating galactose units contained in the carrageenan. The polymer chains in carrageenan can be cleaved by hydrolytic depolymerization upon treatment with an acid, or by oxidative depolymerization upon treatment with hydrogen peroxide. Upon cleavage of the polymer chains, carrageenan products having optimum gelling properties and viscosity can be obtained.
In a typical process for making pure carrageenan, crude seaweed is first washed with cold water to remove sand and other particulates that may be present after the seaweed has been harvested. Carrageenan typically does not swell during the cold wash, primarily because carrageenan in seaweed is associated with the structural components of the seaweed, generally cellulose. Depending on the seaweed species, following the cold wash a hot water extraction procedure is typically performed in which the extracted carrageenan is treated with aqueous base at high temperature. Generally, the base used is an alkali or alkaline earth metal hydroxide such as, for example, NaOH, Ca(OH)2, or KOH. This high temperature aqueous base modification leads to the formation of 3,6-anhydro linkages in the galactose units of the carrageenan polysaccharide. After base modification, the hot extract is filtered to remove insoluble material such as cellulose, hemicellulose and other particulates, and acid is added to adjust the pH to 9 or lower. The filtrate can then be concentrated to about4% carrageenan for further processing. Optional procedural steps after extraction may include centrifugation and bleaching. Pure carrageenan is typically obtained by precipitation of the extract from the aqueous solution with KCl or an alcohol such as isopropanol.
Preparation of pure carrageenan by extraction on a commercial scale is expensive because of viscosity and gelling properties that limit the rate at which such carrageenan products can be processed. After the extraction step, a hot aqueous stream can typically only contain low concentrations of carrageenan, typically up to about 4%. At higher concentrations of carrageenan, the aqueous stream becomes too viscous to be processed efficiently. Therefore, a relatively low proportion of carrageenan is obtained per unit volume of the aqueous process stream.
There has been an ongoing search for more cost-effective methods of preparing semi-refined carrageenan and other products as lower cost replacements for conventional carrageenan. Semi-refined carrageenan (SRC) products are those in which few or none of the structural components of the seaweed, principally cellulose, have been removed. During the preparation of SRC, a salt such as KCl or NaCl typically is added during base modification along with the base. The presence of sufficient amounts of salt prevents disintegration of the seaweed structure and inhibits extraction of carrageenan from the seaweed. An alcohol, such as isopropanol, can also be used to inhibit extraction during the high temperature base modification step. Following the base modification step, with the seaweed structure still intact, the processed seaweed mixture is typically dried to afford SRC. When the seaweed is a member of the Eucheuma family, the SRC obtained is known as processed Eucheuma seaweed (PES).
U.S. Pat. No. 5,502,179 is directed to processes wherein the seaweed starting (feed) material is mixed with a base and a solvent in which carrageenan is insoluble. This base-treated mixture is washed with an aqueous solution and then subjected to shear stress treatment (such as an extrusion process or by means of a shear mixer). The shear stress treatment is said to be performed on seaweed material having a solids content of at least 25%, at a temperature between iooaC and 175° C., for about 10 to 200 seconds. The carrageenan material resulting from the shear stress treatment may be dried and subjected to comminution, grinding or milling to obtain a powder with a particle size in the range of 0.05 to 0.5 mm.
Conventional carrageenans typically have some advantages relative to SRC obtained from the same seaweed. One such advantage is that they typically begin to hydrate, i.e., they begin to swell and solubilize, at a lower temperatures than does SRC. For example, conventional iota carrageenan obtained from Eucheuma spinosum will hydrate, swell, and solubilize at room temperature. PES derived from Eucheuma spinosum, however, does not hydrate, swell and solubilize until first heated to above about 60° C. Another such advantage is that because SRC contains cellulosic and other materials which are absent in conventional carrageenan, gels formed from SRC have reduced clarity relative to pure carrageenan gels. Therefore, utility of SRC products has been limited to those applications where the food product is not required to be clear and can be readily prepared at the high temperatures required for hydration and solubility of carrageenan to occur. For many applications, however, it is desirable to have an SRC which provides gelling without the need for such high temperatures. It is also important that SRC have acceptable color and appearance suitable for use in products made for human consumption such as, for example, cream, ice cream, other dairy products, and beverages, as well as non-food products such as, for example, cosmetics, tooth paste, and other personal care products.
Therefore, there is a need in the art for a low cost processes for producing carrageenan products with improved hydration characteristics while maintaining acceptable color and appearance suitable for use in food products.